Hydrostratigraphy

Water resources availability depend upon the local geology and can be grouped into one of two hydrostratigraphic types, depending on the permeability of the rock formation.

Aquifers are highly permeable and can therefore store and transmit (under the influence of a pressure gradient) significant quantities of groundwater.
Aquicludes have low porosity and do not store useful quantitities of groundwater: surface water is the main potential in these areas.

Most of Cockpit Country is a limestone aquifer: exceptions being the yellow limestone areas around the Central Inlier (the area from Albert Town, to Crown Lands) and on the west, from Maroon Town through Garlands, Niagara, Elderslie, Accompong to the Quickstep area. The Nassau Valley, to the south, is an Alluvium Aquifer and is the source of the Black River.
The remainder consists of aquicludes, and it is here that surface water, in the form of lakes, rivers and swamps, occurs.

The hydrologic properties of each hydrostratigraphic unit are explained below.

Basal Aquiclude – The rocks characterized regionally as the Basal Aquiclude are essentially those that form the lowest stratigraphic sequence of Jamaica. These are the oldest rocks and they range from Cretaceous to Mid-Eocene (100 mya to 45 mya) in age. Rocks of the basal complex are for the main part comprised of volcanics and volcaniclastics and to a lesser extent the Yellow Limestone Group. These rocks constitute the base level for karst processes in the Cockpit Country.
Limestone Aquifer – The Limestone Aquifer rests unconformably on the Yellow Limestone. The Limestone Aquifer occurs very extensively throughout the Cockpit Country as various subdivisions of the highly karstified White Limestone Group but is comprised mainly of the Troy/Claremont Formation. The age of this formation ranges from Mid-Eocene to Oligocene (45 mya to 30 mya). As a result of the tectonic uplift that occurred subsequent to this period and resulting in the emergence of Jamaica from the sea, the limestone has been deformed, thus making the unit extremely susceptible to karstification, resulting in a complex multitude of joints/fractures (secondary porosity), as well as solution-enlarged conduits. The high permeability of these limestones and the associated high infiltration capacity does not support extensive surface water systems. Instead, most of the rainfall over these areas is immediately channeled into the subsurface, thus establishing well-developed drainage systems. Transmissivity of the Limestone Aquifer is highly variable and unpredictable, as the size and number of conduits will determine its local productivity. This aquifer is highly vulnerable to pollution at any point on the surface.
Limestone Aquiclude – The Limestone Aquiclude forms a down-faulted block to the north of the Cockpit Country, functioning as a subsurface barrier, preventing groundwater flow to the north. This block is part of the Montpelier Formation, typified by chalky limestones, and is Miocene in age (20 mya). The Limestone Aquiclude is not considered to have undergone sufficient karstification to generate any significant levels of permeability. Faults that traverse the Limestone Aquiclude function as preferential groundwater flow paths (i.e. zones of relatively high permeability) only where the fault is linked to significant limestone aquifer storage. This allows for wells to be developed in the Montpelier Formation and structural conditions may be suited to the occurrence of springs.
Alluvium Aquifer /Alluvium Aquiclude – This formation consists of unconsolidated sediments that are formed by the weathering of surface exposed rocks. These sediments have been deposited in association with surface water channels or within interior valleys formed on the limestone. These sediments are all regarded as recent in age (1.8 mya to present).
Perched Watertable - The low permeability of the clay and silts which mantle the floors of the poljes cause the retention of a large amount of the rainfall which falls on the clay itself which is held in the clay itself or in the form of small superficial ponds. In this way a secondary “perched” watertable may be formed above the underlying White Limestone. This perched watertable does not seem to bear any relationship to the water table level in the limestone below.